1. Field of the Invention
The present invention relates to a honeycomb-like regenerative bed element. More particularly, the present invention is directed to a honeycomb-like regenerative bed element having an infinite number of honeycomb-like cells or through-bores formed therein, which is suited for use in a waste heat recovery system provided in a combustion device of a regenerative heating type, such as a regenerative-heating-type radiant tube burner or a regenerative-heating-type open flame burner (i.e. a direct firing regenerative burner, and also may be suited for use in the so-called high-frequency heat regeneration system that repeats the cycle of heat storage and heat emission in a short period, for example, 20 sec. to 60 sec.
It should be understood that the honeycomb-like regenerative bed element mentioned herein is not limited to the one having an infinite number of hexagonal cells by its name, but inclusive of other regenerative bed elements having other and different shapes of cells, such as square cells or triangular cells. 2. Description of Prior Art
In recent years various combustion air preheating techniques have been developed in the field of combustion devices, which recover a substantial amount of heat quantity from an exhaust gas, with a view to increasing thermal efficiency. For instance, although not shown, there has been an alternate combustion type radiant tube burner available, which has regenerative bed element (see Industrial Heating, P 71, Vol. 23, No. 6, Issued by the Japan Industrial Furnace Manufacturers Association, and the U.S. Pat. No. 4,856,492 and 4,870,947). According thereto, a pair of burners are provided at both ends of a radiant tube, respectively, for alternate combustion purposes, and one of a pair of regenerative bed elements is provided equipped at each of those two burners. Hence, a combustion gas is exhausted through the non-operated burner and associated regenerative bed element, while combustion takes place in the operated burner, and then, heat stored in that regenerative bed element is used for preheating combustion air.
With regard to the regenerative bed element, it was attempted to obtain an optimal structure which meets a high waste heat recovery and high thermal efficiency in this field of burner technique.
For example, an attempt was made to utilize a conventional heat storage brick, metallic regenerative materials, or the like, which have been used in industrial furnaces for steel manufacturing or the ceramics industry, as a regenerative means for recovering a sensible heat. In general, a suitable regenerative bed element may be formed by the heat storage bricks of about 6 cm thickness in a grid or cross stripe pattern, or by use of pure metal materials of about 2-3 cm thickness. But, it has recently been proposed to adopt ceramics balls, like an alumina ball, as the regenerative bed element.
The use of such heat storage brick and metallic regenerative materials, however, has been found unsuitable for the reason that the brick is slow in absorbing and emitting heat, requiring some tens of minutes or some hours to repeat the cycle of heat storage and emission and resulting in a low average temperature and a poor thermal efficiency, further the metallic regenerative materials can not be used with such an oxidizable gas as exhaust combustion gas and a high-temperature combustion gas of over 1000.degree. C. because of heat resistance restrictions, despite being suited for the high-frequency regenerative operation (which repeats the cycle of heat storage and emission in a short period of time, as short as 20 to 60 sec.). Consequently, the brick is so great in volume for heat storage as to need a sufficient long time for storing therein or emitting therefrom heat, which is not suited for the rapid heat storage/emission cycle, and the metallic regenerative material can not be used in the high-temperature and oxidation circumstances.
In order to solve the foregoing problems, an attempt was made find the most suitable material which has great specific surface area and heat resistance, and not reactive with a combustion gas. Honeycomb-like ceramics provide an answer to this demand, and in particular, a catalytic ceramics for decreasing NOx in exhaust gas of an automobile was found to be applicable.
None the less, the usefulness of catalytic ceramics resides primarily in having a large surface area to bear therein as much catalyzer as possible, with the result that a person in the art can attempt to make the walls of cells in the ceramics thinner to a minimum degree in order to attain the smallest possible pitch of the cell. But, this structure will increase draft resistance. Thus, the catalytic ceramics, which may work in a high-pressure-ratio engine of an automobile without substantial problem, will raise a problem of pressure loss, if it is applied to the burners or similar combustion devices, such pressure loss is also critical and unsuitable for a regenerative bed element used in combustion devices. One can consider reducing such draft resistance (i.e. pressure loss) by Increasing the cell pitch while maintaining the thickness of cell wall so as to enhance the draft efficiency, but as a result, the storage heat capacity will be reduced. Consequently, the automotive catalytic ceramics is not always best applicable to the regenerative bed element.
For various reasons, it has been difficult to achieve a compact or small-sized regenerative bed element with a small draft resistance and a sufficiently large degree of storage heat capacity and area of heat transfer surfaces. Namely, attempts to obtain the most preferred structure of honeycomb-like regenerative bed element suited for the burners or other similar combustion devices have been unsuccessful.